It is estimated that by 2020 there will be some 33 million automotive vehicles sold annually with built-in wireless connectivity, generating more than 163 million terabytes of data each year via their dozens of on-board cameras and sensor technologies. When shared across a wireless network, this data can be utilized by vehicles to give them an awareness of road conditions beyond the reach of their sensors, and thus enable the driver or the vehicle itself to better plan driving maneuvers. Vehicle to Vehicle (V2V) communications is a subset of device to device (D2D) wireless technology designed to allow automobiles to “talk” to each other.
One configuration of device to device (D2D) and vehicle to vehicle (V2V) communication is shown in FIG. 1, where three vehicles 1, 2 and 3 may be in communication with each other and may also be in communication with a base station such as an LTE base station 4, which may be in communication with another wireless device 5.
Release 12 of the Long Term Evolution (LTE) wireless communication standard has been extended to support device to device (D2D) communications features targeting both commercial and public safety applications. Some applications enabled by Rel-12 LTE include device discovery, where a device is able to sense the proximity of another device and associated applications by broadcasting and detecting discovery messages that carry device and application identities. Another application consists of direct communication based on physical channels terminated directly between devices.
One potential extension for device to device communication includes support of V2x communication (Vehicle to “anything”), where “x” includes any combination of direct communication between vehicles, pedestrians and infrastructure. V2x communications may enable forward collision warning, traffic queue warning, vulnerable road user alerts, do not pass warnings, curve speed warnings, blind intersection warnings, emergency vehicle alerts, etc.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements, e.g., latency, reliability, capacity, etc. European Telecommunication Standards Institute (ETSI) has defined two types of messages for road safety: Co-operative Awareness Message (CAM) and Decentralized Environmental Notification Message (DENM).
CAM: The Co-operative Awareness Message (CAM) is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. The CAM message also serves as active assistance to safety driving for normal traffic. The availability of a CAM message is checked every 100 ms, yielding a maximum detection latency requirement of <=100 ms for most messages. In other words, the CAM must be less than or equal to 100 ms. However, the latency requirement for pre-crash sensing warning is 50 ms.
DENM: The Decentralized Environmental Notification Message (DENM) is event-triggered, such as by braking, and the availability of a DENM message is also checked every 100 ms, and the requirement of maximum latency is <=100 ms.
The package size of CAM and DENM messages varies from 100+ to 800+ bytes and the typical size is about 300 bytes. The message is supposed to be detected by all vehicles in proximity. The SAE (Society of Automotive Engineers) has also defined the Basic Safety Message (BSM) for dedicated short range communication (DSRC) with various messages sizes defined. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.
Link adaptation is used for maximizing the link spectral efficiency for a given propagation condition. Typical link adaptation protocols are closed loop, i.e., they are based on some measurement at the receiving device of transmit signal from a transmitter device and some signaling, e.g., a channel quality index (CQI) feedback or a recommended transmission format, from the receiver device to the transmitter device.
In V2x scenarios, it will often be impractical to establish a closed-loop protocol for dynamic link adaptation. The reasons include the fact that many V2x topologies are one-to-many and the feedback load would be too large. Another reason is that many V2x services require very low latency, incompatible with the latency incurred by a practical feedback protocol.
Therefore, it is unpractical to perform link adaptation in V2x communication based on feedback from receivers.